Describes: a) a set of procedures for the connection-oriented transfer of data, control and reference information between protocol machines, which implement the functions of a provider of the Basic Class Virtual Terminal Service; b) the two modes in which those procedures operate; c) the structure of protocol elements used for the transfer of data and control information and the mapping of these protocol elements onto ACSE and lower layer services; d) the means of negotiating the functional units to be used by the protocol machines and the parameters of the service; e) the structure and mapping of protocol elements used for the transfer of data and control information.